The body of Genelle Conway- Allen, 13, of Suisun City was found Friday in a park.

Photo: Courtesy Of Fairfield Police, Fairfield Police

The body of Genelle Conway- Allen, 13, of Suisun City was found...

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Genelle Renee Conway-Allen, 13, of Suisun City, in a photo taken just before she was last seen Jan. 31, 2013. Her body was found the next day in a park in Fairfield.

Photo: Courtesy Of Fairfield Police, Fairfield Police

Genelle Renee Conway-Allen, 13, of Suisun City, in a photo taken...

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Parents and children from the nearby Sheldon Elementary school pray for the 13-year-old Green Valley Middle School student Genelle Renee Conway-Allen at her memorial in Allan Witt Park in Fairfield, Calif., on Monday, February 4, 2013.

Photo: Liz Hafalia, The Chronicle

Parents and children from the nearby Sheldon Elementary school pray...

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Eric May, a homeless man, said he found the body of 13-year-old Green Valley Middle School student Genelle Renee Conway-Allen at Allan Witt Park in Fairfield, Calif., before alerting passing police.

Photo: Liz Hafalia, The Chronicle

Eric May, a homeless man, said he found the body of 13-year-old...

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Looking at messages at the memorial for 13-year-old Green Valley Middle School student Genelle Renee Conway-Allen on Monday, February 4, who was found dead and naked at Allan Witt Park in Fairfield, Calif.

Photo: Liz Hafalia, The Chronicle

Looking at messages at the memorial for 13-year-old Green Valley...

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The playground was under investigation during the weekend as it remains empty on Monday, February 4, after 13-year-old Green Valley Middle School student, identified by friends as Genelle Renee Conway-Allen, was found dead at Allan Witt Park in Fairfield, Calif.

Photo: Liz Hafalia, The Chronicle

The playground was under investigation during the weekend as it...

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Cheryl Parrish (left) and her aunt Roshawn Forrest (right) holding Coco from Fairfiled on Monday, February 4, at a memorial for 13-year-old Green Valley Middle School student Genelle Renee Conway-Allen who was found at Allan Witt Park in Fairfield, Calif.

Photo: Liz Hafalia, The Chronicle

Cheryl Parrish (left) and her aunt Roshawn Forrest (right) holding...

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Anayah Hyde (left) and Anastasia Keola visit a makeshift memorial at Allan Witt Park in Fairfield, Calif. on Saturday, Feb. 2, 2013, one day after Genelle Renee Conway-Allen's body was discovered in the parking lot Friday morning. Officials are investigating the case as a possible homicide.

FBI agents and U.S. Justice Department officials have joined the investigation into the killing of a 13-year-old girl whose body was found in a Fairfield park, police said Wednesday.

Genelle Conway-Allen of Suisun City was last seen in the area of Railroad Avenue, along the border with Fairfield, the afternoon of Jan. 31. Her body was discovered nearly 4 miles away in Allan Witt Park early the next morning.

Federal agents are often brought into a kidnapping investigation, but Fairfield Police Chief Walter Tibbet declined to characterize Genelle's disappearance and slaying as an abduction because he did not want to "create a panic."

However, Tibbet urged parents to talk to their children about safe behavior in public, including not talking to strangers and shouting for attention if an unknown adult approaches them.

"It's a fine line between vigilance and hypervigilance," Tibbet said.

At the intersection of East Tabor Avenue and Grande Circle in Fairfield, where a surveillance camera took a photo of Genelle shortly before she vanished, Major Market manager Ibrahim Mohammad said he had told police he may have spoken to the girl that afternoon.

Mohammad said he had seen a girl matching Genelle's description standing outside his store and asked her if she was waiting for a ride. She said yes, Mohammad said.

"I can't say for sure if it was her, but maybe it will help police," Mohammad said.

He added that he has had trouble sleeping since Genelle was killed.

"This is not a situation anyone in this community will accept until they find this monster," Mohammad said. "This is our daughter. This happened to all of us."